The present invention relates to a thermal line printer of the type having a divided line head composed of a plurality of linear thermal head segments aligned in the widthwise direction of a recording medium sheet perpendicular to the feeding direction or the lengthwise direction of the recording medium sheet so as to cover the entire span or width of the recording medium sheet.
One type of conventional divided line head is composed of a plurality of linear thermal head segments aligned in a single row and coupled to each other at opposed ends of adjacent segments. Each linear segment has a given length sufficient to cover the span of A4 or B4 size recording paper and is formed with a plurality of heating elements arranged linearly on the segment at a given pitch. These linear head segments are connected in series to each other to constitute the divided line head which can cover the entire length of larger size recording paper such as A1 size and A0 size, and which has a higher yield rate than that of a corresponding monolithic line head of comparative length.
However, this type of conventional divided line head has drawback that the pitch of the heating elements is made irregular along the junction or connecting portion of adjacent segments to thereby impair the quality of the printed image pattern.
Another type of conventional divided line head is disclosed in U.S. Pat. No. 4,660,052. This conventional head is composed of a plurality of linear thermal head segments aligned in a pair of parallel rows in staggered relation and in partially overlapping relation at end portions of the linear segments between the parallel rows so as to completely cover the entire width of recording paper. In operation, the first or upstream row of linear segments is activated to effect a part of the single line printing, and then the second or downstream row of linear segments is shifted in the lengthwise direction of recording paper relative thereto through an interval corresponding to the distance between the parallel rows and is activated to effect the remaining part of the single line printing to thereby complete the single line printing. In such operation, in order to avoid duplicate printing by the overlapping portion of the staggered segments between the first and second rows, a predetermined number of heating elements are blanked during the printing operation at the overlapping portion of the staggered linear segments. The staggered linear segments must be precisely positioned relative to each other to set a predetermined overlapping dimension corresponding to the span of the predetermined number of the blanked heating elements. However, it is practically quite difficult to precisely and equally set the overlapping dimension of the individual staggered segments between the pair of rows due to positioning error during assembling of the divided line head and due to thermal expansion of the linear segments caused during the continuous printing operation.